1. Field of the Invention
This invention relates to a distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
2. Description of the Related Art
A prior-art distance measurement apparatus of a spread spectrum type measures the distance between the apparatus and a target object by using an electromagnetic wave modulated in accordance with a pseudo random noise code (a PN code). Specifically, a beam of an electromagnetic wave whose amplitude is modulated in accordance with a PN code of a predetermined bit length (a predetermined chip length) is emitted in a forward direction with respect to the body of the apparatus. A moment of the transmission of the PN code with the electromagnetic wave is memorized. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a target object. The received echo beam is converted into a corresponding electric signal. The echo-beam-corresponding electric signal is binarized into a bi-level echo electric signal. Calculation is made about the value of the correlation between the bi-level echo electric signal and the PN code used for the modulation of the transmitted electromagnetic wave. A moment at which the calculated correlation value peaks is detected as a moment of the reception of the PN code contained in the echo beam. The prior-art apparatus calculates the distance between the apparatus and the target object from the time interval between the moment of the transmission of the PN code and the moment of the reception thereof.
An example of the electromagnetic wave is light emitted from a laser diode. In this case, the prior-art apparatus uses a photodiode or a phototransistor as a photodetector (a photoelectric conversion device) for converting a received echo beam into a corresponding electric signal. The photodetector outputs the electric signal. The voltage of the electric signal outputted from the photodetector varies only in a positive side or a negative side of a circuit reference potential generally equal to a circuit ground potential. In the prior-art apparatus, binarizing the output signal of the photodetector is implemented by comparing the voltage of the output signal of the photodetector with a threshold voltage (a decision reference voltage). To accurately binarize the output signal of the photodetector, it is necessary to properly set the threshold voltage with respect to a range in which the voltage of the output signal of the photodetector varies. Inaccurately binarizing the output signal of the photodetector reduces the accuracy of the calculated distance to a target object.
Japanese patent application publication number 5-264724 discloses a distance measurement apparatus using a radio SS (spread spectrum) signal. The apparatus in Japanese application 5-264724 is divided into a transmitter and a receiver. The transmitter includes a PN generator, a carrier generator, a multiplier, and an RF (radio frequency) portion. The PN generator outputs a PN code to the multiplier. The carrier generator outputs a carrier to the multiplier. The multiplier executes multiplication between the PN code and the carrier, thereby subjecting the carrier with phase modulation responsive to the PN code. The multiplier outputs the modulation-resultant signal to the RF portion. The RF portion converts the output signal of the multiplier into a radio SS signal. The RF portion feeds the radio SS signal to a transmission antenna. The transmission antenna radiates the radio SS signal as a forward signal. The receiver includes an RF portion, a correlator, a detection and wave-shaping portion, a time calculator, and a distance calculator. The PN generator in the transmitter outputs a reference bit signal in the PN code to the time calculator, thereby starting the time calculator counting pulses of a clock signal. A reception antenna receives an echo radio SS signal. The received echo radio SS signal is fed from the reception antenna to the receiver RF portion. The receiver RF portion derives an echo PN code from the received echo radio SS signal. The receiver RF portion outputs the echo PN code to the correlator. The correlator calculates the correlation between the echo PN code and a reference PN code which is the same as the PN code outputted from the PN generator in the transmitter. The correlator generates an autocorrelation waveform signal in response to the calculated correlation. The autocorrelation waveform signal has an amplitude which is maximized when the echo PN code comes into agreement with the reference PN code. The correlator outputs the autocorrelation waveform signal to the detection and wave-shaping portion. The detection and wave-shaping portion subjects the autocorrelation waveform signal to a detection process. The detection and wave-shaping portion outputs the detection-process-resultant signal to the time calculator. The time calculator suspends counting pulses of the clock signal in response to the output signal of the detection and wave-shaping portion. Specifically, the time calculator suspends counting when the echo PN code comes into agreement with the reference PN code. The number of pulses counted by the time calculator indicates the time interval from the transmission of the PN code to the reception of the PN code. The time calculator informs the distance calculator of the time interval. The distance calculator computes, from the time interval, the distance to a measured object reflecting the forward radio SS signal and causing the echo radio SS signal.
Japanese patent application publication number P2000-338243A discloses a coherent laser radar apparatus including a CW laser device and a first optical coupler. The CW laser device outputs a source laser beam to the first optical coupler. The first optical coupler divides the source laser beam into a first sub light beam and a second sub light beam. The first sub light beam propagates from the first optical coupler to an optical modulator. The optical modulator modulates the first sub light beam in accordance with a pseudo random signal (a PN code signal) outputted from a PN-code generator. The modulation-resultant light beam propagates from the optical modulator to an optical antenna before being emitted from the optical antenna as a forward light beam. The forward light beam reaches a target, being scattered and reflected thereby and forming an echo light beam. The echo light beam reaches the antenna. The echo light beam travels from the antenna to a second optical coupler. The second sub light beam propagates from the first optical coupler to a frequency shifter. The frequency shifter changes the frequency of the second sub light beam to generate a local light beam. The local light beam propagates from the frequency shifter to the second optical coupler. The second optical coupler mixes the echo light beam and the local light beam. The second optical coupler outputs the mixing-resultant light beam to an optical detector. The optical detector subjects the mixing-resultant light beam to optical heterodyne detection, thereby generating a beat signal between the echo light beam and the local light beam. The optical detector outputs the beat signal to a correlator. A variable delay device receive the pseudo random signal from the PN-code generator. The variable delay device defers the pseudo random signal by a variable time to generate a delayed pseudo random signal. The variable delay device outputs the delayed pseudo random signal to the correlator. The correlator calculates the correlation between the beat signal and the delayed pseudo random signal. The correlator outputs a correlation-representing signal to a signal processor. The PN-code generator feeds the signal processor with information about the pseudo random signal. The variable delay device informs the signal processor of the signal delay time provided thereby. The signal processor analyzes the strength and frequency of the correlation-resultant signal in response to the information about the pseudo random signal and the signal delay time, thereby detecting the target and the Doppler frequency. The PN-code generator changes the output pseudo random signal among a plurality of different pseudo random bit sequences.
U.S. Pat. No. 6,218,982 B1 corresponding to Japanese patent application publication number P2000-121726A discloses a distance measurement apparatus in which a pseudo random noise code is generated synchronously with a reference clock signal. A first forward electromagnetic wave is transmitted in response to the pseudo random noise code. A first echo wave is received which is caused by reflection of the first forward electromagnetic wave at an object. The received first echo wave is converted into a binary signal. A value of a correlation between the binary signal and the pseudo random noise code is repetitively calculated at a predetermined period having a synchronous relation with the reference clock signal. A time interval taken by the first forward electromagnetic wave and the first echo wave to travel to and from the object is measured in response to a timing at which the calculated correlation value peaks. Then, a second forward electromagnetic wave is transmitted in response to a transmitted pulse signal. A second echo wave related to the second forward electromagnetic wave is received. The received second echo wave is converted into a received pulse signal. A delay circuit defers the transmitted pulse signal by a delay time corresponding to the measured time interval to generate a delayed transmitted pulse signal. A phase difference between the received pulse signal and the delayed transmitted pulse signal is measured at a resolution higher than a resolution corresponding to the predetermined period of the correlation-value calculation. A distance to the object is calculated on the basis of the measured time interval and the measured phase difference.
Japanese patent application publication number 4-363687 discloses a distance measurement apparatus including a carrier generator and a PN-code generator. The carrier generator outputs a carrier to a modulator. The PN-code generator outputs a PN code to the modulator. The modulator modulates the carrier in accordance with the PN code to generate a modulation-resultant baseband SS signal. The modulator outputs the baseband SS signal to an up converter. The up converter changes the baseband SS signal into a corresponding RF SS signal. The up converter feeds the RF SS signal to a transmission antenna. The transmission antenna radiates the RF SS signal as a forward signal. The forward signal is reflected by an object before returning as an echo signal. A reception antenna receives an echo RF SS signal. The received echo RF SS signal is fed from the reception antenna to a down converter. The down converter changes the echo RF SS signal into a corresponding echo baseband SS signal. The down converter outputs the echo baseband SS signal to a demodulator. A variable delay device receives the PN code from the PN-code generator. The variable delay device defers the PN code by a time which increases in accordance with the lapse of time. The variable delay device outputs the delay-resultant PN code to the demodulator. The demodulator implements demodulation responsive to the echo baseband SS signal and the delay-resultant PN code to detect the correlation therebetween. A signal processor detects when the correlation peaks. Information about the signal delay time currently provided by the variable delay device is fed to the signal processor. The signal processor detects the signal delay time which corresponds to the timing at which the correlation peaks. The signal processor computes the distance to the reflecting object on the basis of the detected signal delay time.
It is an object of this invention to provide an accurate distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
A first aspect of this invention provides a distance measurement apparatus comprising code generating means for repetitively generating a pseudo random noise code signal of a predetermined chip length in synchronism with a clock signal of a fixed period to generate a succession of the pseudo random noise code signals; transmitting means for generating a distance-measuring electromagnetic wave in accordance with the succession of the pseudo random noise code signals generated by the code generating means, and for transmitting the generated electromagnetic wave toward an object as a forward electromagnetic wave; receiving means for receiving an echo electromagnetic wave caused by reflection of the forward electromagnetic wave at the object, and for converting the received echo electromagnetic wave into a corresponding received signal which varies only in one of (1) a positive side and (2) a negative side of a reference potential; signal processing means for removing components from the received signal generated by the receiving means to generate a processing-resultant signal, the removed components having frequencies lower than frequencies of components of the pseudo random noise code signal generated by the code generating means; binarizing means for comparing the processing-resultant signal generated by the signal processing means with a preset decision reference voltage to convert the filtering-resultant signal into a corresponding binary signal; correlation value calculating means for sampling the binary signal generated by the binarizing means into received data in synchronism with the clock signal, and for calculating a value of a correlation between the received data and the pseudo random noise code signal generated by the code generating means; distance calculating means for calculating a time interval taken by the electromagnetic wave to travel a distance to the object in forward and backward directions on the basis of the correlation value calculated by the correlation value calculating means, and for computing the distance to the object from the calculated time interval; and transmission start timing controlling means for, before start of the sampling by the correlation value calculating means, causing the code generating means to repetitively generate the pseudo random noise code signal during a surplus time corresponding to at least a stabilization time taken by the received signal to stabilize in direct-current voltage level after start of reception of the echo electromagnetic wave by the receiving means, and thereby for causing a timing of start of transmission of the forward electromagnetic wave by the transmitting means to be earlier than a timing of start of calculation of the correlation value by the correlation value calculating means.
A second aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the transmission start timing controlling means comprises means for, before start of the sampling by the correlation value calculating means, causing the code generating means to repetitively generate the pseudo random noise code signal during the surplus time equal to the stabilization time plus a maximum round-trip time taken by electromagnetic wave to travel a maximum measurable distance in forward and backward directions.
A third aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the correlation value calculating means comprises (1) means for periodically sampling the binary signal to generate received data sampled bits whose number corresponds to the predetermined chip length, and (2) means for calculating the value of the correlation between the received data sampled bits and the bits of the pseudo random noise code signal while shifting the received data sampled bits relative to the pseudo random noise code on a 1-bit by 1-bit basis, and wherein the distance calculating means comprises means for calculating the time interval taken by the electromagnetic wave to travel the distance to the object in the forward and backward directions on the basis of a phase difference between the received data sampled bits and the bits of the pseudo random noise code signal which corresponds to a moment when the correlation value calculated by the correlation value calculating means peaks.
A fourth aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the transmitting means comprises (1) a light emitting element for generating light as the distance-measuring electromagnetic wave and (2) a drive circuit for driving the light emitting element in accordance with the pseudo random noise code signal, and the receiving means comprises a light receiving element for receiving the echo electromagnetic wave caused by reflection of the forward electromagnetic wave at the object.
A fifth aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the signal processing means comprises an amplifier which includes filtering means for removing components from the received signal to generate the processing-resultant signal, the removed components having frequencies lower than the frequencies of components of the pseudo random noise code signal.
A sixth aspect of this invention provides a distance measurement apparatus comprising first means for repetitively generating a pseudo random noise code signal of a predetermined chip length to generate a succession of the pseudo random noise code signals; second means for generating and emitting a forward light beam in response to the succession of the pseudo random noise code signals generated by the first means; third means for converting incident light into a corresponding voltage signal, the incident light including an echo light beam caused by reflection of the forward light beam at an object; a comparator for comparing the voltage signal generated by the third means with a preset decision reference voltage to convert the voltage signal into a corresponding binary signal; a correlator for calculating a correlation between the binary signal generated by the comparator and the pseudo random noise code signal generated by the first means; and fourth means for inhibiting the correlator from responding to the binary signal during a specified time which follows a moment of start of the repetitive generation of the pseudo random noise code signal by the first means, and which covers a time taken by the voltage signal to stabilize in direct-current voltage level.
A seventh aspect of this invention provides a distance measurement apparatus comprising first means for repetitively generating a pseudo random noise code signal of a predetermined chip length to generate a succession of the pseudo random noise code signals; second means for generating and emitting a forward light beam in response to the succession of the pseudo random noise code signals generated by the first means; third means for converting incident light into a corresponding voltage signal, the incident light including an echo light beam caused by reflection of the forward light beam at an object; a high pass filter for subjecting the voltage signal generated by the third means to a high pass filtering process to convert the voltage signal into a filtering-resultant signal; a comparator for comparing the filtering-resultant signal generated by the high pass filter with a preset decision reference voltage to convert the filtering-resultant signal into a corresponding binary signal; and a correlator for calculating a correlation between the binary signal generated by the comparator and the pseudo random noise code signal generated by the first means.
An eighth aspect of this invention is based on the seventh aspect thereof, and provides a distance measurement apparatus further comprising fourth means for inhibiting the correlator from responding to the binary signal during a specified time which follows a moment of start of the repetitive generation of the pseudo random noise code signal by the first means, and which covers a time taken by the voltage signal to stabilize in direct-current voltage level.